When you are grading homeworks and the timer ticks all the way down to 0:00 you don't get a new homework like you used to but just keep the homework up front with the timer standing still at 0:00.
That way you "block" the homework to be graded because no one else will get it as the system still sees you grading it. This further slows down the whole grading system, as some might just want to check out the grading and don't really bother to write a feedback anytime soon.
That way many homeworks might be laying around for a while until the assistant teachers will grade it.
I don't know if the bug is on every WoX-Site but I discovered this one on the German WoP-Site.
Customer support service by UserEcho